This invention concerns a method for the printing of packaging parts. In connection with dry offset printing of packaging parts, particularly plastic lids, it is commonly known to feed the lids individually via carriers/chain cassettes, in which the individual lid parts are placed. These are fed forward to a printing section where the dry offset printing takes place. The carriers are disposed on chains at a distance corresponding to about the length of two lids. Normally, there are 20 to 30 carriers for conveying the lids from the insertion stacks and forward to the restacking. Before the printing, there is thus performed a punching out of the lids and a stacking operation. The actual printing is effected by feeding a single lid forward with the carriers to the printing section, where a printing pad is fed down on to the surface of the lid to deposit its print. The packaging is then conveyed further and, after having been dried, a stacking takes place.
This known technique thus suffers the disadvantage that it involves two stacking operations, and also that the printing is carried out on the individual lids when these are fed forward in cassettes. With the known methods, the 30 lids are mounted singly in cassettes from which they are hereafter removed again. This means that a lot of mechanics are involved in the form of stacking, and re-stacking the cassettes. In practice, this means that the stability is not satisfactory when the speed needs to be high. Consequently, the process is inexpediently resource demanding, time-consuming and costly.
It is known from 3,539,085 to guide foils in the form of webbing through, for example, a printing press, and where among other things the control system takes into account and controls the tensions in the webbing by controlling the speed of the rollers. The method does not solve the problem involved in the placing of print on the packaging parts, which packaging parts are not produced in one plane and are consequently unsuitable for the mass production as described in the publication.
From WO 91/15342 there is known an apparatus and a method for the placing of irregularly demarcated pictures on a foil. The publication does not solve the problem involved in the clear control of the individual parts which are desired to be provided with print, and consequently does not permit a mass production in connection with the packaging parts.
Another problem with the printing of lids is that it is difficult to attain high line speeds due to inaccuracies in the positioning of lids on the foil web. According to the prior art, corrections must be made to adjust for these positioning inaccuracies, with consequent delays in production, as the foil web must be adjusted to assure accurate printing on the parts. This may generally require the foil web to move backwards as well as forwards with consequent delays in production.